Potentiostat can be defined as amplifiers that are used to maintain a constant voltage between the working electrode and the reference electrode by making current controlled adjustments at the counter electrode. [5]
The main function of the potentiostat include
Measurement of the potential difference between the reference electrode and the working electrode
To compare the potential difference with the pre defined voltage and forces the current through the working electrode to oppose the difference between the predefined voltage and the existing voltage across the working electrode [5].
The potentiostat behaves as a controlling and measuring device .It controls the voltage across the electrochemical cell by detecting the changes in the resistance and accordingly varying the current that is provided to the cell. The relation between the current and the resistance can be defined by Ohm’s law as,[17]
where, [17]
R is the variable resistance
I is the output current of the potentiostat and,
E is the constant potential.
When the resistance is high the current is reduced and vice versa, so that a constant potential is maintained. [17].
A basic standardised potentiostat circuit consists of an input differential amplifier which compares the voltage between the reference electrode and working electrode to a desired biased working voltage. The error voltage obtained is amplified and is supplied to the counter electrode. The impedance change between the reference electrode and working electrode will affect the voltage supplied to the counter electrode so that constant potential is maintained between the reference and the working electrode. The output voltage is obtained which is proportional to the electrochemical cell by connecting the voltage follower circuit to the working electrode.The current follower hold the counter electrode to virtual ground. The potentiostat maintains the required reference -working voltage by comparing the reference electrode voltage to the required biased potential and making adjustments in the potential at the working electrode.[34]
The circuit of the conventional Potentiostat circuit is as shown in the figure below.[34]
Figure 1.Basic potentiostat circuit [34]
Where ,
C = counter electrode of the electrochemical cell
R=reference electrode of the electrochemical cell
W=working electrode of the electrochemical cell
Rf= feedback resistance
V= Voltage given as ,
V=I sensor/Rf [34]
CA = Controlled amplifier
APPLICATIONS:
The applications of potentiostat include
Potentiostat as Ammeter with zero resistance, controlled voltage and current source
Measurement of polarised resistance and red ox voltages
Potentiostat as a High Fidelity Amplifier
Differential voltage control
Biosensors [35].
FEATURES:
The special features of the potentiostat such as speed, stability in phase, O/P power, and high input resistance play a very important role in most of the electrochemical processes.[35]
Significant features of potentiostat are listed below.
Accuracy and wide ranges in potentials and currents.
High file size, scanning and sampling rate
Current resolution
Series of working electrodes for controlling instruments
Sweep analog generator and footprints.[35]
A basic potentiostat setup is as shown in the figure above. i(t) represents the current carrying with respect to time(t).CE,RE and WE represents counter electrode, reference electrode and working electrode respectively.
The potentiostat consists of controlling and amplifying part.
Controlling part: This part maintains the voltage of the electrochemical cell at a required voltage level. The voltage between the RE (reference electrode) and WE(working electrode)is maintained to be equal to the Bias voltage. This is done by sourcing or driving the current into CE(counter electrode).[37]
Amplifying part: This part converts the current of the counter electrode into equivalent voltage [37].
An input potential is applied to the potentiostat through a voltage source (ramp generator, PIC programmed to generate voltage waveform) or by the potentiostat internally. In the project we apply the time variant input hence we use external source. A PIC is used in the project to generate time varying potential which is applied as input to the potentiostat. The potential across the working and the reference electrode is controlled by the potentiostat [19]
The voltage applied produces current through the counter electrode which produces a desired potential across the reference and working electrode.
The basic potentiostat circuit for the electrochemical cell with three electrode setup is as shown below [17].
The input potential Ein is applied to the non inverting terminal of the opamp1 .The output of the op-amp 1 is given through the counter electrode and a feedback is given from the reference electrode through the op-amp 3.This feedback will help to reduce the difference between the inverting and non-inverting inputs of the opamp1 and thus the reference voltage will have same voltage as of Ein. The potential at the working electrode is set to the same potential that was applied to input of opamp1 since the potential difference between the reference and working electrode is zero.[17]
The reference electrode is connected to the Ein pin of the opamp1 through opamp3 of the high impedence.The current needs to flow through the counter electrode and not through the reference electrode due to high resistance and also due to the fact that it may result in uncertainty of its potential.[17]
The electrochemical cell consists of 3 electrodes immersed in an electrolyte solution namely,
Working electrode
Reference electrode and ,
Counter electrode.[1]
The working electrode (WE) is the electrode which is used to analyse the electrochemical reaction. The second electrode that is used in closing the circuit is called the counter electrode (CE).The third electrode is the reference electrode that is used to measure the potential between the working electrode and the electrolyte. [2]
The experimental setup for an electrochemical cell is as shown below.
This setup contains a working electrode/electrolyte junction and the counter electrode/electrolyte junction. Assuming loss of voltage to be negligible in the electrolyte and the solid bulk, the voltage controlled by the battery and that applied to the system can be distributed as,
Where,
VWE and VCE are the potential across the working electrode/electrolyte and counter electrode/electrolyte respectively.
This implies that the voltage supplied by the battery is not applied entirely to the working electrode/electrolyte junction.[2]
The voltage across the counter electrode will vary with time as a electrochemical reaction is stimulated by the current at the interface of the counter electrode/electrolyte(VCE)This instability will affect consistency of the voltage across the working electrode/electrolyte VWE .[2].This inconsistency can be avoided by varying the value of VCE such that VCE<< VWE and this is done by increasing the area of the counter electrode. This results in the resistivity of the Counter Electrode to be smaller as compared to the Working Electrode and thus the condition VCE<< VWE is satisfied.[2]
Choosing a very large area for the CE is always not the best solution since a significant influence is caused by VCE and VWE when high current density flows through the cell. Potentiostat can be used which controls the potential across the Working electrode and the reference electrode directly. The voltage that was supposed to be applied between the reference electrode and the solution is applied as input potential to the potentiostat [2].
There are many sensitive and information based electro analytical techniques in the field of electrochemistry that provide useful information about chemical and physical properties of analytes such as potential ,transfer of electrons,oxidation,coefficients of diffusion which are very difficult to be obtained using other techniques.
The electro analytical methods applied for quantitative measurements are,.
1. Potentiometry: Measurement of the potential in the electrochemical cell under no current flow. Examples include pH measurements, titration etc.[22][23]
2. Coulometry:Measurement of the current which is flowing through the circuit by keeping the potential constant at the working electrode.Examples:determination of thickness of film etc.[24]
3.Electrolytic method:
In this method an external energy source is applied to initiate an electrochemical reaction. If the external energy source that is applied is potential then the current that is resulting is the analytical signal and if the current is adapted then the analytical signal is the potential that is obtained. [22][30].
The approach which uses the applied potential is termed as voltammetry method and those which use current are termed as galvanostatic methods. [22][30]
In the project the voltammetry method (Cyclic voltammetry) is applied, i.e., the external applied driving force is the applied potential. [22][30]
It is an electro analytical process which is used in analytical chemistry. In voltammetry the information of the analyte in the electrochemical cell is obtained by varying the potential with respect to time and measuring the resultant current.[24]
There are various types of Voltammetry such as linear sweep, staircase, Squarewave,cyclic, anodic stripping, cathodic stripping, polarography, differential pulse voltammetry etc.[24]
In this project we use cyclic voltammetry for electrochemical and for electrochemical analysis.
Cyclic voltammetry is one of the widely used methods for analysis of electrochemical reactions. It has ability to provide rapid information about the red ox processes and chemical reactions such as red ox potential peak location, effects on the red ox processes due to media and determination of electron-transfer kinetics.[25][26]
In Cyclic voltammetry (CV), a triangular voltage waveform is applied at the working electrode linearly and the resulting current is measured .The figure below shows the triangular potential waveform applied.
Where, Ef= final potential
Ei= initial potential
Cycle 1, 2 .. = every cycle constitutes of forward and reverse scan.
For every cycle, the potentiostat measures the current generated due to the varying potential applied. The graph obtained by plotting the potential against the resulting current is termed as cyclic voltammogram which is dependent on time and on various chemical and physical aspects.
Figure below shows the cyclic voltammogram for a single cycle of the red ox species.
Where,
Epc = peak cathode potential
Epa=peak anode potential
Ipc=peak cathode current
Ipa=peak anode current
It is presumed that initially there is only oxidised form in the red ox species. Hence in the first half cycle negative potential scan is considered where the cathodic peak current is observed. In the next half cycle positive potential scan is considered and the anodic peak current is observed. The main parameter taken into consideration in the cyclic voltammogram are the peak potentials and the peak current at the anode and cathode sweeps.[25]
The peak current ip is directly proportional to the concentration C and can be defined by Randles-Sevcik equation,
ip = 0.4463 n F A C (n F v D / R T)1/2[25]
where,
n = number of electrons transferred/molecule
A = electrode surface area (cm2)
C = concentration (mol cm-3)
D = diffusion coefficient (cm2 s-1).
The Block diagram of the Potentiostat is as shown below:
The PIC microcontroller is used for the generation of the voltage waveform, data analysis and data acquisition. The PIC is programmed in the C language as it is the more efficient and powerful coding technique. A 10 bit counter is used in the PIC microcontroller for the voltage waveform generation.[9]
The 10 Bit counter value is fed into external D/A converter through the SPI (serial programming interface) port of the microcontroller. The voltage waveform is level shifted using the amplifier circuit and is applied to the electrochemical cell between the working and the reference electrode of the cell.[9]
The current generated from the cell is then converted into voltage and is then applied to the level shifting circuit which is used to achieve the required input voltage level that is essential for the analog input port of the PIC.The current readings are noted down for each incrementing value of the voltage.[13][9].
The Potentiostat circuit is designed using a PIC which controls the operation of the circuit. The circuit diagram of the Potentiostat circuit with PIC18F46K20 is as shown below.
The components used in the schematic are as below:
Item
Quantity
Reference
Part
1
4
C1,C3,C4,C5
100uF/Electrolytic
2
1
C2
2×4.7uF/Ceramic
3
1
J1
CON6
4
9
R1,R2,R4,R7,R8,R10,R11,R12,R14
R
5
3
R3,R9,R13
POT
6
1
R5
2K/0.25W
7
1
R6
1.33K/0.25W
8
1
U1
PTN78000A
9
1
U2
PTN04050
10
1
U3
LM148
11
1
U4
PIC18F46K20
12
1
U5
TLC5615
13
1
U6
SW
14
1
U7
OSC
15
1
U8
LM741/DIP8
PTN78000A: PTN78000A is a high efficiency integrated switching regulator which operates through a wide range of frequency, providing voltage conversion that ranges from negative to positive loads up to 1.[4]
PTN04050:It is an integrated switch regulator with adjustable output. The input operating voltage ranges from 2.9-7V.The output voltage ranges over a wide range of values (-15V to -3.3V) and is determined by using a single external resistor .This regulator provides a very efficient positive to negative voltage conversion for loads up to 6W.[7]
LM148: LM148 is an op-amp (operational amplifier) that has high gain and low offset current and bias. Each package of LM148 consists of four amplifier isolated from each other. This reduces the overall cost and the complexity of the circuit. The package is supplied with power rather than individual op-amps with ± 12V.[10].
TLC5615:It is a 10 bit Digital to analog converter is a product of Texas instruments with an input supply of 5V used in operations where space is critical. It uses a network of resistors which is buffered with Op-amp (gain A=2) for A/D conversions. Both the input and the output polarity of the LC5615 are same .[11]
LM741/DIP8: it is a low cost operational amplifier which has protection of excess load at the input and output .the LM741 is free from oscillations. The operating temperature ranges from 0°C- 70°C.[12].
PIC18F46K20:PIC18F46K20 is a high performance flash programming microcontroller from Microchip with optimised C complier architecture. It consists of 10 bit ADC module used for analog to digital conversion, 16 bit instruction set,8 bit data path, and interrupts with priority levels. The operating voltage ranges from 1.8V-3.6V.[31].
A/D Module setting
Timer Module Setting
SPI Module Setting
Interrupt Enable and Data collection
The Circuit operation is as explained below:
A triangular potential waveform is generated by the PIC18F46K20 using a up down counter
The output of the PIC is in the digital form and is converted to analog from using a digital to analog converter(D/A converter) using the SPI port of the PIC.The D/A converter using in this circuit is MAX5354[3]
The variable output from the digital to analog converter is dependent of the Vref pin of the D/A converter i.e., MAX5354 .The varying output of the D/A converter is controlled by using a potentiometer which is connected to the Vref pin.[3]
The output of the D/A converter which is unipolar is converted into bipolar by using an op-amp level shifting circuit. Op amps are used to compare adc voltage and feedback voltage from electro chemical cell. A buffer is introduced between the voltage that is level shifted and counter electrode so that loading of previous op-amp phases obtained as a result of the current drawn from the electrochemical cell is prevented.[3]
The non inverting terminal of the voltage follower circuit is connected to the reference electrode so that a constant voltage is maintained by drawing very less current.[3]
The inverting terminal of the current to voltage converter is connected to the working electrode to ground the working electrode and also to obtain an equivalent voltage of the current (drawn from electrochemical cell) at the voltage converter output.[3]
The converted current as a voltage is amplified and level shifted so that the voltage is brought between the acceptable PIC voltage ranges (ie,0-5V) and is given as input through the analog port of the PIC[3].
Con6 in the circuit diagram is applied with +5V voltage externally. Pin 1, 2 and 3 of con6 are connected to the amplifiers and the output pin is connected to the electrochemical cell .Con6 provides +5V input to U2,where U2 boosts the signal from 5V to 12V that is necessary for circuit operation. Input is given to U2 from U1 which produces -12V output needed for the circuit operation.U5 does the analog to digital conversion when it gets a potential waveform from the PIC and the converted values are given as input to the amplifier circuitry.
LEVEL SHIFTING CIRCUIT:
The circuit below shows the level shifting circuit present in the Figure 3.
The output of the TLC5615 (D/A Converter) is connected to the level shifting circuit. Level shifting output Vout .The level shifting output voltage Vout is a function of the output of D/A converter and the offset DC voltage and is defined by the mathematical equation as shown below.[3]
Vout= -(R4/R2) (Offset DC Voltage) + (1+R4/R2)(O/P of TLC5615) .[3]
In this project we use R4=R2=1kohm
Hence,
Vout= -1(Offset DC Voltage)+2(O/P of TLC5615) .
The output of the level shifting circuit Vout is the representation of the current that is given to the electrochemical cell. The range of voltages can be obtained by varying the R5 (potentiometer) which is connected to the ref pin of TLC5615 and the voltage polarity can be varied by changing the potentiometer R2 of the offset DC voltage.[3]
The output of the level shifting circuit is as shown.
The RC equivalent circuit of an electrochemical cell is as shown .the components of the circuit is as explained below.
Rrc is the resistance between the counter electrode and the reference electrode .It is defined as the solution resistance which is the most important factor of Z(impedance) in the electrochemical cell. Rrc depends on the ion concentration the nature of ions and the area in which the current is passed.[14]
The resistance Rrc can be defined as,
Rrc = r*l/A [14]
Where, r is the is the density ,l is the length which carries a constant current and A, is the area in which the current is passed.[14]
Cd is the double layer capacitance .This capacitance is present at the electrode and the electrolytic solution interface. This is formed when the ions from the electrolyte solution resides on the surface of the electrode .the Cd depends on the type of ions, concentration of the ions, temperature, layer of oxidation, impurities present in the solution etc.[14]
Rs is the uncompensated resistance of the solution. It sometimes include the component present on the surface of the metal.[15].If the uncompensated resistance is very high then the separation between the voltage peaks is more and there is significant decrease in the peak currents.
Rd is the resistance across the interface of the metal and the solution.[15]
The analog-to-digital measurements are periodically taken from the special event mode of the TC mode (timer capture mode) by setting the go/done bit of the A/D converter which starts the analog-digital measurements. Once this is done an interrupt of high level is applied. [3]
The stages involved in the interrupt subroutine are as shown below. [3]
A/D values read from the conversion register
Continuous 8 values averaged
( to eliminate noise)
Store the data in an array of length=256
Disable interrupts after 2048 counts
Once the interrupt is disabled the peak current of the anodic and cathodic sweeps are obtained.
The slope of the left side and the knee is calculated. Then the peak of both anodic and cathodic sweeps with respect to the slope is calculated. The calibration data of the curve is stored in the PIC in terms of slope and y-intercept of the best fit line. This slope and the y-intercept are used to calculate the concentration that corresponds to the peak current values and the corresponding 8 bit data value of the current is generated. [3]
The point of maximum curvature is used to determine the baseline current for both anodic and cathodic sweeps. A line is made to pass through all the points to the left side of the point of maximum curvature. [3]
The novel method (L method) finds the edge/boundary between the pair of lines that fits the cure closely. This method considers all points of data together at same time and is used where the curve includes sharp jumps. [27]
The linearity of the region to the right and left of the line is taken into consideration in the L method. If a line is drawn fitting all the points on the left hand side and another line is drawn on the right hand side, the area that is obtained in between these two lines will be same as the area of the knee. To find these 2 lines, a pair of line that fits the curve well (i.e., that covers maximum points) is obtained. [27]
Consider a graph of no of data v/s evaluation graph. Let the number of data be’d’.Hence the data range is from ‘1-d’.Let ‘p’be the partitioning of the data points. Hence the right hand side of the data points is defined to be from 1..p and the left side of the data points from p+1…d.[27]
RMSEc= 1/d {p * RMSE (Lc) + (d-p) * RMSE (Rc)} [27]
Where,
RMSEc= the mean square error in total
RMSE (Lc) =the mean square error if the left hand side of the partition in total
RMSE (Rc)= the mean square error if the right hand side of the partition in total.
Steps to find the knee of the curve using the L method is as shown below.[28]
The data points are varied from 1 to d.
Using the points on the right hand side(1 to p), a line is fit and using the points on the left (p+1 to d),another line is fit.
The RMSEc is calculated for each value of p.
The knee of the curve is obtained by the least mean square error.
The regressive line fitting is used for fitting a line to the set of data provided.
The best fit line(regression line) is used to obtain minimum value for the mean square error(i.e., the error between the expected and the obtained values if the data points).[29]
If the data points are defined by {xi,yj} then the regression line s used to obtain the equation of a straight line[29]
Y=mx+c
Where, m=slope
c =y-intercept.
The values for the m and c can be obtained by using calculus and is given as, [28]
m= covariance (xy) / {variance (x) – mean(x) mean(y)}
c = a * mean(x) – mean(y)
The peak is obtained w.r.t the baseline current and the concentration is obtained from the calibration curve. [29]
MPLAB IDE v8.53 and MCC18 complier are used to program and load the program onto the PIC.PIC2 development debugger/programmer is used for interfacing.
MPLAB IDE v8.53 is the high optimised and fast complier from Microchip used for PIC18 series of microcontrollers. It is the most useful toolset used in the development of the embedded designs.[32]
MCC18 is the MPLAB C complier which is integrated into the MPLAB IDE .This provides compatibility for the MPLAB to program in C language. It controls the path and the declarations of source files header files, linker files etc that are needed in the programming.[33].
The PORTS of the PIC are configured as inputs and outputs. In this project PORTC and pin 4-7 of PORT D are configured as output and pin1-3 of PORT D as inputs. PORT A behaves as an Analog input port.
The ADC, SPI, TIMER registers for A/D conversion, Serial data transfers is as shown.
ADC Register:
The features of Analog-Digital converter module are:
Auto data acquisition capacity
External channels(13),10 bit resolution
Conversion is processes during sleep
Fixed reference voltage of 1.2V [38]
SETTINGS:
1.ADCON 0 register[38]:
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
CHS3
CHS2
CHS1
CHS0
GO/DONE
ADON
0
0
0
1
1
ADCON – A/D conversion starts when ADON=1 ,this is automatically cleared when the A/D conversion is done [38]
CHS[3:0] : Analog Channel Select bits.[38]
0001= channel 1 is selected.
2.ADCON 2 Register:
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
ADFM
ACQT2
ACQT1
ACQT0
ADCS2
ADCS1
ADCS0
1
0
0
0
0
0
1
ADFM- defines the format in which the A/D conversion values are stored.
ADFM=1 stores the values as right justified i.e. ADRESL = b7-b0 and ADRES=b9 and b8.
ADCS[2:0]- select bits for A/D clock conversion
ADCS[2:0]=001 selects FOSC/8.[38]
3.SPI (Serial Port interface)[38]:
SPI module is used to transfer / receive synchronous data of 8 bits simultaneously between the microcontrollers or peripheral devices such as A/D converters , shit registers ,EEPROM etc.[38]
The master SSP module has 4 registers assigned for SPI operation. These are:
• SSPCON1 – register for CONTRL
• SSPSTAT – register to define STATUS
• SSPBUF – Buffer to send the data in serial/parallel
• SSPSR – register for shift.[38]
SSPSTAT register (status register)[38]:
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
SMP
CKE
D/A
P
S
R/W
UA
BF
X
0
X
X
X
X
X
X
CKE – Clock bit select for SPI
CKE =0, the data O/P changes when the clock transition takes place from idle to active.[38].
4.SSPCON1 Register (control register)[38]
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
WCOL
SSPOV
SSPEN
CKP
SSPM3
SSPM2
SSPM1
SSPM0
1
X
1
1
0
0
0
0
WCOL- Write collision detect ,
WCOL = 1, the SPI buffer register is written and data is simultaneously transferred.
SSPEN- synchronous serial port enable data
SSPEN= 1 ,Serial port are enabled and pins are configured
CKP-Polarity of the clock
CKP=1 the clock is idle when it is at high
SSPM3[2:0] – selection of synchronous serial port mode
SSPM3[2:0]= 0000, Master SPI mode and the clock frequency = FOSC/4.[38]
Timer and CCP (capture compare PWM) register:
The PIC14F46K20 consists of two registers each consisting of 16 bits. Each of the register can function as PWM /capture/compare register.[38]
5.CCP2CON (CCP Control register)[38]
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
DC2B1
DC2B0
CCP2M3
CCP2M2
CCP2M1
CCP2M0
X
X
X
X
1
0
1
1
CCP2M[3:0]- Mode selection register
CCP2M[3:0]= 1011,SPI event trigger,A/D conversion, starts when COP2IF bit is set
6.T3CON (Timer 3 Control Register)[38]
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
RD16
T3CCP2
T3CKPS1
T3CKPS0
T3CCP1
T3SYNC
TMR3CS
TMR30N
1
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